Process for making high flexural strength paper



United States Patent Jersey No Drawing. Filed May 20, 1959, Ser. No. 814,409

2 Claims. (Cl. 162163) This invention relates to a method for increasing the flexural strength of fibrous cellulosic products by incorporating therein a polyethylene polyamine lignosulfonate and to the improved fibrous products obtained thereby.

Stiffness, without accompanying brittleness, is a desirable attribute of paperboard products such as cartons used in the protective packaging of various commodities. Such cartons must have adequate stiffness to insure proper operation on packaging machinery, to retain the packaged product without undue deformation of the package and to give the customer a feeling of sturdiness and strength of package rather than flirnsiness and shoddiness.

Stiffness of paper and paperboard products may be increased by an increase in thickness of the sheet, but it is often desirable to increase stiifness without increasing the caliper, or thickness, of the packaging material, both from the standpoint of economy and machine workability. Thus, the packaging industry desires high ilexural strength in paperboard packaging materials. Flexural strength is a measure of stifiness independent of the thickness, or caliper, of the sheet material.

I have found that the flexural strength (i.e. stiffness per unit caliper) of paper and paperboard may be increased by the precipitation of insoluble polyethylene polyamine salt of an anion such as the lignosulfonate, alkyl aryl sulfonate or alkyl naphthalene sulfonate in the presence of an aqueous slurry of cellulose fiber pulp which occludes the freshlyprecipitated polyethylene polyamine salt. Paper sheets made from pulp so. treated have greater stiifness per unit of thickness than paper made from untreated pulp. Polyethylene polyamine lignosuL fonates are the preferred stiffening agents, although polyamine salts of other anions may also be used as will be detailed later.

The process of my invention, as illustrated by treatment of cellulose pulp with polyethylene polyamine lignosulfonate, may be carried out as follows: An aqueous slurry of cellulose pulp obtained from wood, straw or similar fibrous material by any conventional digestive cooking process such as the sulfite process, the soda process, the sulfate process or other similar process is admixed With an aqueous solution of lignosulfonic acid or a soluble metal salt of lignosulfonic acid. To the pulp slurry is then added sufiicient polyethylene polyamine to precipitate the lignosulfonate present in the pulp slurry as a water-insoluble polyethylene polyamine lignosulfonate salt. The pulp slurry is then made into a paper sheet on conventional paper-making equipment. The insoluble polyamine lignosulfonate becomes largely trapped or occluded by the pulp fibers and remains fixed on the fiber surface throughout the paper-making process. When the paper web formed from the pulp on conventional equipment is passed over the drying rolls, the precipitated polyethylene polyamine lignosulfonate which is present fuses and then sets as a thermosetting resinous material under the influence of the drierheat and thereby contributes markedly to the fiexural strength of the resulting paper sheet.

The polyethylene polyamines, in order to be satisfactory for my use, must be water soluble but also must form insoluble salts of lignosulfonate and other suitable anions. The lower molecular weight polyethylene polyamines such as ethylene diamine, diethylene triamine and triethylene tetrainine form soluble lignosulfonate salts and hence are 3,180,787 Patented Apr. 27, 1965 not suitable. In general, polyethylene polyamines having molecular weights in excess of are satisfactory, and those ranging in molecular weight from 500 to 1000 are preferred.

Lignosulfonates satisfactory for use in my invention may be obtained by isolation from spent sulfite liquor by any of several means. For example, treatment of spent sulfite liquor with lime according to the Howard process described in the United States Patents Re. 18,268, Decemher 1, 1931; 1,856,558, May 3, 1932 and 1,924,361, August 29, 1933, results in the recovery of lignin material as basic calcium lignosulfonate. This water-insoluble material may be converted to the water-soluble lignosulfonic acid or soluble salt derivative such as the ammonium or alkali metal salt by known methods. These water soluble compounds are suitable for use in our invention.

I have also found that separation of the lignosulfonate fraction from spent sulfite liquor is not a necessary prerequisite to its use in my invention, since the whole spent sulfite liquor may be economically utilized in my process. Whole spent sulfite liquor may be added to a slurry of cellulose pulp and, upon addition of a water solution of the desired polyethylene polyamine, the lignosulfonate content of the liquor is precipitated as an insoluble polyethylene polyamine lignosulfonate and becomes largely occluded by the pulp fibers. On making paper from this pulp slurry, the soluble wood sugars, carbohydrates and hemicellulose present in the whole spent liquor are separated from the pulp on the paper machine wire and suction rolls and thereby pass otf in the white water, leaving only the desired insoluble polyethylene polyamine lignosulfonate atiixed to the fibers of the paper web.

The degree of increase in flexural strength resulting from the process of my invention is dependent both on the amount of the polyethylene polyamine lignosulfonate, alkyl aryl sulfonate or alkyl naphthalene sulfonate oceluded with a given weight of pulp and on the composition of the insoluble polyamine salt. Ihave found that higher amounts of the salt result in greater flexural strength up to a maximum which is obtained when b tween about 30% and 50% of polyethylene polyamine lignosulfonate or similar insoluble polyethylene polyamine salt based on the weight of the pulp fibers is present. Higher percentages fail to increase the flexural strength further, and may even result in a decrease in flexural strength due to the increased brittleness of the sheet. Furthermore,fthe1 amount offlexural strength increase. will vary depending on the ratio of polyethylene polyamine to lignosulfonate or other anion used in the preparation of the insoluble salt. Both polyethylene polyamines and lignosulfonic acid have a plurality of reactive salt-forming groups so that these two reactants may be combined in varying ratios to form salts. In general, an increase in the polyamine to lignosulfonate ratio results in increased fiexural strength of the treated paperboard except when both polyamine and lignosulfonate are added in relatively large amounts.

The data in the following Table I indicate the increase in flexural strength of paperboard obtained by precipitating varying amounts of polyethylene polyamine lignosulfonate on pulp using a constant ratio of the amine and lignosulfonate reactants, whereby a constantcomposition of the precipitated polyethylene polyamine lignosulfonate is obtained. The samples were prepared and tested as follows: I

Nine grams of sulfite cellulose pulp were dispersed in 900 cc. of water. To a pulp slurry of this composition was added varying amounts of spent sulfite liquor and then polyethylene polyamine as indicated in Table I. The

spent liquor contained about 10 gm. of organic solids of which about 5 gm. comprised lignosulfonates calculated as polyamine had a molecular weight of about 650 and was added as a 10% aqueous solution. The treated pulp was 'formed into a 9" X 9" handsheet by conventional procedures, which included drying the sheet at a temperature (about 200 F.) sufiicient to fuse the precipitated amine lignosulfonate on the fibers of the sheet, and the flexural strength was obtained on this handsheet. Plexural strength is obtained by subjecting a strip of paperboard to sufiicient longitudinal bending stress to fracture or separate a sufiicient number of the fibers so that a crease forms across the width of the board. The force necessary to bend the board to its creasing or breaking point is measured and fiexural strength is calculated using this value. In the present case, a l-inch wide strip of the handsheet about 4 inches long was positioned horizontally on two vertical supporting columns so as to span the 1 /8 inch gap between the supports. Force was applied downward on the test sample at the midpoint of the 1% inch span by a steel knife /J, inch in thickness having a rounded edge. The force applied to the knife blade was measured by a strain gauge, the varying resistance of the gauge being read from a bridge amplifier meter calibrated in ounces. The data thus obtained were used to calculate fiexural strength by the following formula:

S=?:PL/32WT in which Flexural strength calculated by the above formula is independent of the dimensions of the sheet and thus is indicative of the stiffness of a sheet of unit thickness.

TABLE I Flexural strength hand sheets from pulp treated with varying amounts of polyethylene polyamine lignosulfonate Spent sulfite Polyethylene Polyamine ligno- Flexural liquor solids 1 polyamine 1 sultonate in strength finished board 1 in p.s.i.

0 None (Control) 2, 300

1 Parts per 100 parts of oven dry pulp.

It will be noted that as the amount of polyethylene polyamine lignosulfonate is increased, there is first obtained a substantial increase in fiexural strength which reaches a maximum when the amine lignosulfonate is present to the extent of about 40% of the weight of pulp used. At higher concentrations, flexural strength drops due to the increased brittleness of the sheet. Significant increases in fiexural strength are obtained when the amount of polyethylene polyamine lignosulfonate is from about 5% to about 80% of the dry' weight of the pulp. We prefer the range between 30% and 50% of the pulp weight.

As previously mentioned, the composition of the polyethylene polyamine lignosulfonate as controlled by the ratio of amine to lignosulfonate also has a significant effect on flexural strength.

In the following Table II are presented fiexural strength data on handsheets prepared and tested in the manner previously described in which the pulp slurry was treated with varying amounts of spent sulfite liquor and polyethylene polyamine of about 650 M.W. as shown in the table.

4 TABLE n Flexural strength of handsheets from pulp treated with polyethylene polyamine lignosulfonate Flexural strength, p.s.i.

Parts polyamine 0 2 4 10 14 Parts SSL solids;

1 Parts per parts oven dry pulp.

' with the amine lignosulfonate salt that brittleness results,

accompanied by a lowering in fiexural strength.

Significant increases in fiexural strength have been obtained when as little as 1 part of polyethylene polyamine and 10 parts of spent sulfite liquor solids (added as spent sulfite liquor) are added to a slurry of 100 parts of pulp. We prefer to treat a slurry of 100 parts of pulp with from 2 to 10 parts of amine and from 10 to 150 parts of spent sulfite liquor solids, higher loadings being less economical and the highly loaded pulp being more diflicult to handle on the paper machine.

Similar results may be obtained by the use of other water-soluble polyethylene polyamines having molecular weights ranging upward from about 150, although as the lower limit of molecular weight is approached, the amount of amine must be increased somewhat to compensate for greater solubility of the amine lignosulfonate salts and hence lessened efliciency of these amines as lignosulfonate precipitants.

Further indications of the efficaey of my invention were obtained by treatment of pulp according to our process followed by formation of the treated pulp into paperboard on a paper-making machine of pilot plant scale; In a typical test, a 1% aqueous slurry of the following solids composition was formed into a 12 inch wide paperboard web on a continuous pilot plant scale paper-making machine. Drying of the web was carried out under conditions such that the web temperature reached about 200 F, suflicient to fuse the precipitated polyethylene polyamine salt to the fibers of the web. A furnish of this composition results in paperboard which is suitable for use as the inner liner portion of cylinder board.

Parts Repulped waste news 400 Repulped miscellaneous paperboard including bleached sulfite and kraft stocks 600 Starch 50 Dicalite 1 50 Dicalite is a diatomaceous earth sold by the Dicalite Corporation.

Adjustments were made to produce lb. per ream (3000 sq. ft.) paperboard at a speed of about 25 feet per minute. A quantity of untreated board was first prepared as a control. The remainder of the pulp slurry was then treated with 12.5 parts of spent sulfite liquor solids (added as desugared spent sulfite liquor) and 2 parts of polyethylene polyamine of 650 molecular weight for each 100 parts of oven dried pulp present in the slurry. Paperboard made from this treated pulp was then compared with untreated board with the results shown in Table III.

TABLE III Flexural strength Board Board in p.s.i.

basis thickness, Mullen, weight, in. p.s.i. lbs/rm. With Cross grain grain Untreated pulp 121 0170 31 1, 790 1, 300 Treated pulp 120 0169 49 2, 520 1, 700

The data in Table III indicate that treatment of pulp according to the process of this invention resulted in a paperboard having an increase in the with grain flexural strength of over 40% and in cross grain flexural strength of over 30%. Furthermore, the Mullen or bursting strength of the paperboard, as measured by the standard TAPPI method, T483M, was increased by 58%. It is thus evident that very significant increases in flexural strength and bursting strength of paperboard result from treating the cellulose pulp with polyethylene poly-amine and lignosulfonates in accordance with the procedure herein described.

The order in which the lignosulfonate solution and the polyethylene polyamine are added to the pulp slurry is of no consequence, good results being obtained irrespective of which reagent is added first. If the lignosulfonate is to be added as a solid salt, however, it is preferable that it be added first and completely dissolved in the pulp slurry prior to the addition of the polyethylene polyamide.

Although lignosulfonates are preferred anions for use in this invention due to their low cost and availability, other polyanions which react with polyethylene polyamines to form water insoluble salts having properties similar to those of the polyethylene polyamine lignosulfonates previously described may also be used. Alkyl aryl sulfonates such as dodecyl benzene sulfonate, condensed alkyl naphthalene sulfonates, soluble polystyrene sulfonates and sulfonated phenol-formaldehyde resins are representative suitable anion types.

The degree to which flexural strength of products from cellulose pulp may be increased by use of the above types of non-ligueous anions in the process of this invention may be seen from the following examples.

Separate 10 gm. portions of the typical furnish for cylinder board inner liner previously described on column 4 of this specification were slurried in 900 ml. portions of water and treated as shown in Table IV with a 10% solution of polyethylene polyamine having a molecular weight of about 650 and with the sodium salts of the polyamine. precipitable polyanions indicated. Handsheets were prepared from the treated pulps and flexural strengths compared to handsheets from an untreated, control pulp slurry.

TABLE IV Flexnral strength of handsheets from pulp treated with polyethylene polyamine and condensed alkyl phenol sulfonates and alkyl naphthalene sulfonates "The particular material used in this case was a sulfonated phenoliormaldehyde resin described in detail in Chemistry and Technology of Leather, Vol. 2, page 349 (Reinhold Publishing 00., New York, 1958).

2 The particular material used in this case was a sulfonated naphthalene-lorrnaldehyde condensate described in detail in SmfaceActive Agents, page 120 (Interscience Publishers Inc, New York, 1949). It is available commercially from the Rohm and Haas Company under the trade name Turnol N.

As is evident from the above data, the polyethylene polyamine salts of the sulfonated phenol-formaldehyde resin and sulfonated naphthalene resin increased the flexural strength of the test sheets by 32% and 41%, respectively. Similar results may be obtained by substituting other polyanions which are precipitable by polyethylene polyamine.

The anion should be chosen so that the insoluble polyethylene polyamine salt formed by reaction between the amine and the anion is fusible at moderately elevated temperatures, preferably below C. Thus, when the salt is intimately co-mingled with the cellulose pulp by precipitation in situ in the pulp slurry, the subsequent drying operations will result in fusing the salt so that it coats and adheres tenaciously to the individual cellulose fibers. solidification of the salt either by cooling to crystallization or as a thermosetting resin results in the development of the desired increase in flexural strength.

I have found the use of lignosulfonate as the anion for manufacture of the fiber-reinforcing agents of my invention to be of particular value for the formation of the inner plies of cylinder board and in similar paperboard stock in which a pure white color of the finished product is considered to be unnecessary, since the insoluble polyethylene polyamine lignosulfonates are brown in color and thereby impart a tan cast to the paper in which they are incorporated. For many uses, whiteness is of secondary importance to flexural strength, and my invention is particularly valuable for such uses. If pure whiteness is desired, one of the lighter colored anions herein described may be substituted for lignosulfonate, or a layer of bleached pulp may be superimposed on the product of my invention, in a manner commonly used in the manufacture of cylinder board, thereby obtaining, in large measure, the fiber reinforcement of my invention coupled with the surface whiteness contributed by the untreated pulp.

In addition to increasing the flexural strength of paper and paperboard, my invention is of value in increasing the stiffness and flexural strength of molded cellulose pulp products including paper plates and eating utensils and products of the papier-mach type.

Having now described and illustrated by example pre ferred forms of my invention, it will be obvious that various modifications can be made without departing from the spirit thereof. Therefore, no limitations on my invention are intended except as specifically set forth in the appended claims.

I claim:

1. A method for increasing the flexural strength of paper which comprises adding a water soluble lignosulfonate salt to a water slurry of cellulose pulp in an amount between 10% and of the dry Weight of said pulp, adding a polyethylene polyamine having a molecular weight in excess of 150 in an amount between 1% and 10% of the dry weight of said cellulose pulp thereby precipitating insoluble polyethylene polyamine lignosulfonate in intimate admixture with said cellulose pulp, and forming the pulp fibers with adhered insoluble precipitate into a continuous paper web.

2. A method which comprises adding a water-soluble lignosulfonate salt to a water slurry of cellulose pulp in an amount'between 10 and 150 percent of the dry weight of said pulp, adding a polyethylene polyamine having a molecular weight in excess of 150 in an amount between 7 1 and 10 percent of the dry weight of said cellulose pulp, thereby precipitating an insoluble polyethylene polyamine lignosulfonate in intimate admixture with such cellulose pulp, forming the pulp fibers with adhered insoluble precipitate into a felted mat, and heating said mat at elevated temperatures to dry the same and to cure said polyethylene polyamine lignosulfonate.

References Cited by the Examiner UNITED STATES PATENTS 2,297,635 9/42 Schorger 162-463 Neubert et al. 162-464 Daniel 162-164 Keim 162-167 Muller 260-1243 Goss 26G17.5

DONALL H. SYLVESTER, Primary Examiner.

RICHARD D. NEVIUS, JOSEPH B. SPENCER,

MORRIS O. WOLK, Examiners. 

1. A METHOD FOR INCREASING THE FLEXURAL STRENGTH OF PAPER WHICH COMPRISES ADDING A WATER SOUBLE LIGNOSULFONATE SALT TO A WATER SLURRY OF CELLULOSE PULP IN AN AMOUNT BETWEEN 10% AND 150% OF THE DRY WEIGHT OF SAID PULP, ADDING A POLYETHYLENE POLYAMINE HAVING A MOLECULAR WEIGHT IN EXCESS OF 150 IN AN AMOUNT BETWEEN 1% AND 10% OF THE DRY WEIGHT OF SAID CELLULOSE PULP THEREBY PRECIPITATING INSOLUBLE POLYETHYLENE POLYAMINE LIGNOSULFONATE IN INTIMATE ADMISTURE WITH SAID CELLULOSE PULP, AND FORMING THE PULP FIBERS WITH ADHERED INSOLUBLE PRECIPITATE INTO A CONTINUOUS PAPER WEB. 